1. Field of the Invention
The present invention relates to a translucent polycrystalline sintered body suitable for the manufacture of an arc tube for a high-intensity discharge lamp (HID: High Intensity Discharged lamp) such as a high-pressure sodium vapor lamp or a metal halide lamp. The invention further relates to a method for producing the translucent polycrystalline sintered body and an arc tube for high-intensity discharge lamp containing the translucent polycrystalline sintered body.
2. Description of the Related Art
A method described in Japanese Patent No. 2780941 is known as a technology for improving the load capacity of a sintered ceramic body for a high-intensity discharge lamp, thereby improving the load capacity of the discharge lamp. The load includes the thermal wall load applied to the sintered body in operation of the discharge lamp and the wall temperature of the sintered body used as the discharge tube.
The method described in Japanese Patent No. 2780941 has been accomplished based on the finding that when three additives MgO (magnesium oxide), ZrO2 (zirconium oxide), and Y2O3 (yttrium oxide) are used in combination, the resultant sintered ceramic body can exhibit significantly more excellent properties due to the interaction between the three components as compared with conventional ceramic bodies to which the additives are added not in the combination of the above three.
In conventional methods, the combination of MgO and ZrO2 or the combination of MgO, Y2O3, and La2O3 (lanthanum oxide) is added, and the additives are used in relatively large amounts. However, the combination use of MgO, Y2O3, and La2O3 cannot produce satisfactory results, and rather the conventional ceramic bodies using the combination are inferior in qualities to ceramic bodies using only MgO.
In Japanese Patent No. 2780941, the problem of the conventional methods is solved by simultaneously adding the three additives (MgO, ZrO2, and Y2O3) in small amounts. The amount (weight ratio) of MgO is 100 to 800 ppm, preferably 100 to 600 ppm, particularly preferably 150 to 280 ppm, the amount of ZrO2 is 200 to 1200 ppm, preferably 200 to 800 ppm, particularly preferably 300 to 600 ppm, and the amount of Y2O3 is 10 to 300 ppm, preferably 10 to 150 ppm, particularly preferably 20 to 75 ppm.
Meanwhile, a translucent polycrystalline sintered body is suitable for the manufacture of an arc tube for a high-intensity discharge lamp such as a high-pressure sodium vapor lamp or a metal halide lamp. It is desired that the translucent polycrystalline sintered body is high in total light transmittance and mechanical strength (particularly bending strength). A translucent alumina (a sintered alumina body with translucency) has such characteristics, and thereby has been widely used for an arc tube.
However, in a high-intensity discharge lamp, the arc tube of the conventional translucent alumina is generally poor in in-line transmittance due to grain boundary birefringence, etc. A 0.5-mm-thick flat plate composed of the conventional translucent alumina has an in-line transmittance of less than 30%. A light generated by discharge between electrodes is scattered in the arc tube, and then emitted to the outside. Thus, the size of the light source is limited by the arc tube, and the arc tube acts not as a point light source but as a diffused light source. In a case where the discharge lamp is used in combination with a reflecting mirror in a store lighting, a projector light source, a car headlight, etc., the arc portion is placed and operated in a focal position or the like to obtain the optimum light distribution characteristics. When a light is emitted from a position other than the focal position, the desired light distribution characteristics cannot be obtained, resulting in optical design difficulty. It is difficult to control the light from the diffused light source by using the reflecting mirror or lens. Therefore, it is difficult to use the above discharge lamp in combination with the reflecting mirror in an optical device, and the use of the discharge lamp is limited to general lighting application.
In contrast, in a case where a translucent alumina having a high in-line transmittance in at least the visible region is used for an arc tube, a light generated by discharge between electrodes is directly emitted straight. When the arc tube has a small discharge distance, it can be utilized substantially as a point light source. The light from the point light source can be optically controlled by using a reflecting mirror or lens in combination therewith. For example, the light may be converted to a parallel light and may be collected in a spot.
The in-line transmittance may be improved by increasing the firing temperature, thereby increasing the average grain diameter of the sintered body. However, in this case, only some of the grains are grown, and a mixed grain structure containing fine grains and coarse grains is formed, which causes the in-line transmittance to be lowered. In addition, when the average grain diameter of the sintered body is excessively increased, bending strength deterioration and intergranular cracking (grain separation observed in growth process) may be caused disadvantageously.
The in-line transmittance may be improved also by extremely reducing the grain size to prevent the light scattering on the grain boundary (see, Japanese Laid-Open Patent Publication No. 2006-160595). However, in this case, the differences between the in-line transmittance values at various wavelengths are increased, thereby resulting in irregular color in the transmitted light. In addition, the material is fired at a lower firing temperature in this case, so that the resultant sintered body contains highly active grains and shows a low heat resistance unsuitable for the high-intensity discharge lamp for high-temperature use.